NAME | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

MQ_OVERVIEW(7)            Linux Programmer's Manual           MQ_OVERVIEW(7)

NAME         top

       mq_overview - overview of POSIX message queues

DESCRIPTION         top

       POSIX message queues allow processes to exchange data in the form of
       messages.  This API is distinct from that provided by System V
       message queues (msgget(2), msgsnd(2), msgrcv(2), etc.), but provides
       similar functionality.

       Message queues are created and opened using mq_open(3); this function
       returns a message queue descriptor (mqd_t), which is used to refer to
       the open message queue in later calls.  Each message queue is
       identified by a name of the form /somename; that is, a null-
       terminated string of up to NAME_MAX (i.e., 255) characters consisting
       of an initial slash, followed by one or more characters, none of
       which are slashes.  Two processes can operate on the same queue by
       passing the same name to mq_open(3).

       Messages are transferred to and from a queue using mq_send(3) and
       mq_receive(3).  When a process has finished using the queue, it
       closes it using mq_close(3), and when the queue is no longer
       required, it can be deleted using mq_unlink(3).  Queue attributes can
       be retrieved and (in some cases) modified using mq_getattr(3) and
       mq_setattr(3).  A process can request asynchronous notification of
       the arrival of a message on a previously empty queue using
       mq_notify(3).

       A message queue descriptor is a reference to an open message queue
       description (cf.  open(2)).  After a fork(2), a child inherits copies
       of its parent's message queue descriptors, and these descriptors
       refer to the same open message queue descriptions as the
       corresponding descriptors in the parent.  Corresponding descriptors
       in the two processes share the flags (mq_flags) that are associated
       with the open message queue description.

       Each message has an associated priority, and messages are always
       delivered to the receiving process highest priority first.  Message
       priorities range from 0 (low) to sysconf(_SC_MQ_PRIO_MAX) - 1 (high).
       On Linux, sysconf(_SC_MQ_PRIO_MAX) returns 32768, but POSIX.1-2001
       requires only that an implementation support at least priorities in
       the range 0 to 31; some implementations provide only this range.

       The remainder of this section describes some specific details of the
       Linux implementation of POSIX message queues.

   Library interfaces and system calls
       In most cases the mq_*() library interfaces listed above are
       implemented on top of underlying system calls of the same name.
       Deviations from this scheme are indicated in the following table:

              Library interface    System call
              mq_close(3)          close(2)
              mq_getattr(3)        mq_getsetattr(2)
              mq_notify(3)         mq_notify(2)
              mq_open(3)           mq_open(2)
              mq_receive(3)        mq_timedreceive(2)
              mq_send(3)           mq_timedsend(2)
              mq_setattr(3)        mq_getsetattr(2)
              mq_timedreceive(3)   mq_timedreceive(2)
              mq_timedsend(3)      mq_timedsend(2)
              mq_unlink(3)         mq_unlink(2)

   Versions
       POSIX message queues have been supported on Linux since kernel 2.6.6.
       Glibc support has been provided since version 2.3.4.

   Kernel configuration
       Support for POSIX message queues is configurable via the
       CONFIG_POSIX_MQUEUE kernel configuration option.  This option is
       enabled by default.

   Persistence
       POSIX message queues have kernel persistence: if not removed by
       mq_unlink(3), a message queue will exist until the system is shut
       down.

   Linking
       Programs using the POSIX message queue API must be compiled with cc
       -lrt to link against the real-time library, librt.

   /proc interfaces
       The following interfaces can be used to limit the amount of kernel
       memory consumed by POSIX message queues:

       /proc/sys/fs/mqueue/msg_max
              This file can be used to view and change the ceiling value for
              the maximum number of messages in a queue.  This value acts as
              a ceiling on the attr->mq_maxmsg argument given to mq_open(3).
              The default value for msg_max is 10.  The minimum value is 1
              (10 in kernels before 2.6.28).  The upper limit is HARD_MAX:
              (131072 / sizeof(void *)) (32768 on Linux/86).  This limit is
              ignored for privileged processes (CAP_SYS_RESOURCE), but the
              HARD_MAX ceiling is nevertheless imposed.

       /proc/sys/fs/mqueue/msgsize_max
              This file can be used to view and change the ceiling on the
              maximum message size.  This value acts as a ceiling on the
              attr->mq_msgsize argument given to mq_open(3).  The default
              value for msgsize_max is 8192 bytes.  The minimum value is 128
              (8192 in kernels before 2.6.28).  The upper limit for
              msgsize_max is 1,048,576 (in kernels before 2.6.28, the upper
              limit was INT_MAX; that is, 2,147,483,647 on Linux/86).  This
              limit is ignored for privileged processes (CAP_SYS_RESOURCE).

       /proc/sys/fs/mqueue/queues_max
              This file can be used to view and change the system-wide limit
              on the number of message queues that can be created.  Only
              privileged processes (CAP_SYS_RESOURCE) can create new message
              queues once this limit has been reached.  The default value
              for queues_max is 256; it can be changed to any value in the
              range 0 to INT_MAX.

   Resource limit
       The RLIMIT_MSGQUEUE resource limit, which places a limit on the
       amount of space that can be consumed by all of the message queues
       belonging to a process's real user ID, is described in getrlimit(2).

   Mounting the message queue filesystem
       On Linux, message queues are created in a virtual filesystem.  (Other
       implementations may also provide such a feature, but the details are
       likely to differ.)  This filesystem can be mounted (by the superuser)
       using the following commands:

           # mkdir /dev/mqueue
           # mount -t mqueue none /dev/mqueue

       The sticky bit is automatically enabled on the mount directory.

       After the filesystem has been mounted, the message queues on the
       system can be viewed and manipulated using the commands usually used
       for files (e.g., ls(1) and rm(1)).

       The contents of each file in the directory consist of a single line
       containing information about the queue:

           $ cat /dev/mqueue/mymq
           QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260

       These fields are as follows:

       QSIZE  Number of bytes of data in all messages in the queue.

       NOTIFY_PID
              If this is nonzero, then the process with this PID has used
              mq_notify(3) to register for asynchronous message
              notification, and the remaining fields describe how
              notification occurs.

       NOTIFY Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2
              is SIGEV_THREAD.

       SIGNO  Signal number to be used for SIGEV_SIGNAL.

   Polling message queue descriptors
       On Linux, a message queue descriptor is actually a file descriptor,
       and can be monitored using select(2), poll(2), or epoll(7).  This is
       not portable.

CONFORMING TO         top

       POSIX.1-2001.

NOTES         top

       System V message queues (msgget(2), msgsnd(2), msgrcv(2), etc.) are
       an older API for exchanging messages between processes.  POSIX
       message queues provide a better designed interface than System V
       message queues; on the other hand POSIX message queues are less
       widely available (especially on older systems) than System V message
       queues.

       Linux does not currently (2.6.26) support the use of access control
       lists (ACLs) for POSIX message queues.

EXAMPLE         top

       An example of the use of various message queue functions is shown in
       mq_notify(3).

SEE ALSO         top

       getrlimit(2), mq_getsetattr(2), poll(2), select(2), mq_close(3),
       mq_getattr(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3),
       mq_unlink(3), epoll(7)

COLOPHON         top

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2009-09-27                   MQ_OVERVIEW(7)